There are many methods of active balancing series stacked battery cells or super capacitors. Of these methods there is a sub group often described as switched capacitor balancing. There are two well known methods that use switched capacitor type balancing, the first of which is used by Pascual [1], Schmidt [2], Lundquist [3] and others. This method essentially has capacitors that are alternately connected across two adjacent cells using a set of switches. The charge is shuffled via the capacitor from one cell to the adjacent cell. The problem with this system is that if charge has to be transferred from a high State Of Charge (SOC) cell to a low SOC cell that are separated by one or more cells from each other, then the charge has to be shuffled through every cell in between them. This means the charge must flow through all the cell switches and capacitors in between the high SOC cell and low SOC cell. For the current to flow from one cell to the next, there must be a voltage difference between them. If this current has to flow through a number of cells, then this voltage difference will be multiplied by the number of cells it has to flow through. For example, if 100 mV is required to obtain 1 A of current flow between two cells, then if 1 A was required to flow from cells that were spaced 10 cells apart in a stack, there would have to be 100 mV drop between each cell, giving a total of a 1V drop over the ten cells. Hence during the balancing, there will be voltage gradients across cells which is not ideal. This voltage gradient will appear across cells between a high SOC cell and a low SOC cell, in effect unbalancing all the cells in between these two cells which is also not ideal. The voltage gradient will considerably reduce the amount of balancing current that can flow between widely spaced cells. In addition, every time the energy is shuffled from one cell to its adjacent cell, there is some energy loss and hence shuffling charge through all interconnecting cells increases energy losses. However, an advantage of this system is the switches only have to be rated at the cell voltage and are only switching with one cell's voltage across them.
The second method of switched capacitor balancing attempts to address some of these limitations and is used by Marten [4], Castelaz [5] and others. This method essentially enables a single capacitor to be attached to any cell with a bank of switches. For example, the capacitor could first be attached to a high SOC cell and then directly to a low SOC cell. A disadvantage of this system is the switches must be rated at the full battery pack voltage. In addition, the system will incur fairly high switching losses as there will be on average quite a high voltage across the switches e.g. in the case where it is balancing the top and bottom cell, the switches will be switching with the full pack's voltage across them. If the capacitor is connected cyclically between all capacitors, its balancing rate is considerably lower than the first switched capacitor method (in order of 1/n, where n=number of cells, less than first method). The balancing speed can be increased significantly by making intelligent decisions about which cell to connect the capacitor to, but requires cell voltage monitoring and computing ability.